The invention relates generally to neutron detection systems, and more particularly to neutron detection systems with radiation portal monitors.
In view of current enhanced interest in the prevention of terrorist activity, there is a need for practical and sensitive neutron radiation detectors that can detect fissile materials and other sources of neutron radiation and distinguish them from the presence of medical and industrial radioisotopes and from normally occurring radioactive material. Detection of radioactive materials, particularly those illicitly hidden in the stream of commerce, requires fast screening equipment with neutron detection capability. Direct neutron detection capability has the following advantages over the widely present gamma-ray detection for neutron sources: better accuracy due to the extremely low neutron background and fewer sources of neutron emission in the normal flow of commerce and improved effectiveness because the shielding of otherwise detectable neutron sources may be more difficult than for gamma emitters.
A desirable neutron detector should demonstrate improved functionality and identification performance; it should be easily deployable, and have a low total cost of ownership. Currently used neutron detectors include gas proportional counters or liquid scintillators. Gas proportional counters commonly use a gaseous composition, such as helium-3 (a He isotope, denoted as 3He) or a boron-10 (a B isotope, denoted as 10B) containing gas, e.g., 10BF3. In order to achieve suitable detection sensitivity, a large number of neutron-capture nuclides are needed. Due to the very low atomic density presented by the gaseous composition, a relatively large containment area and high pressures may be required to increase the effective interaction probability of the gaseous composition. The manufacturing and ownership costs of such large gas pressurized detectors can be extremely high. Further, pressurized gas containers are subject to federal code regulations for handling and transport and severely limit the portability of gas proportional counters. Liquid scintillators are also sensitive to gamma radiation leading to gamma ray interference.
Conventional neutron detection approaches using solid-state scintillation typically rely on the optical coupling of a neutron-sensing scintillator material composite to a flat window of a photosensor. However, some of the solid-state neutron-sensing scintillator detectors suffer from gamma-ray interference and others suffer from self-absorption of the emitted light and/or optical attenuation of the emission photons before detection.
In order to improve the total neutron sensitivity of the detector, an optimal neutron detector must address the issue of packaging a larger area of neutron-sensing composite and better transportation of the resultant light from the neutron-sensing composite without appreciable gamma-ray interference.